A color television camera generates red (R), green (G), and blue (B) color signals, each having the full visual-signal bandwidth (about 4.2 MHz). These signals are not usually transmitted directly to the television receiver. Instead, a matrix circuit converts the original signal components into a luminance signal (Y) and two chrominance signals (R-Y, B-Y). Part of the reason these signals are used is to make color television signals compatible with monochrome receivers which are capable of responding only to "black and white" luminance signals.
The luminance signal includes the full visual-signal bandwidth and defines the image's brightness (white or gray levels). The chrominance (chroma) signals define the image's color hues (tint) and saturation (intensity) levels. It should be noted here that the terms chrominance or color may be used interchangeably to refer both baseband and modulated signals.
The chrominance signals do not have to include full visual-signal bandwidth. In fact, the bandwidths of the two chrominance signals can be reduced to about 1.6 and 0.6 MHz, respectively, by low-pass filters. Initially, it was hoped that any negative effects from the reduced chrominance bandwidth would be indistinguishable to the human eye.
It has turned out, however, that the smaller bandwidth limitation imposed on the two chrominance signals results in perceptible distortion of the color transitions (i.e., chroma transients) that are produced in the resultant color television display. In general, transient distortion results from a slower rise or fall time associated with chroma transients because of the narrower chroma bandwidths. When the transients are between regions of saturation colors, the distortions can be severe, and the effects can be observed in normal broadcasts.
There have been numerous attempts to improve transient distortion in television signals, examples of which are found in U.S Pat. Nos. 4,030,121; 4,181,917; 4,183,051; 4,245,239; 4,296,433; 4,316,215; 4,414,564; 4,729,014; 4,935,806; 4,979,228; 5,077,603; 5,124,786; and 5,146,319. The full disclosure of each of the above patents is incorporated herein by reference.
Many of the known approaches to controlling transient distortion concentrate on analog solutions, for example U.S Pat. Nos. 4,030,121; 4,181,917; 4,183,051; 4,245,239; 4,296,433; 4,316,215; 5,077,603; and 5,124,786. Although some of these techniques have made considerable advances, most analog circuitry in this area has proven to be rather complicated, costly to implement, and difficult to control.
U.S. Pat. No. 5,146,319 to Engel, U.S. Pat. No. 4,935,806 to Rabii, and U.S. Pat. No. 4,414,564 to Hitchcock are directed toward improvements in a standard signal processing technique known as peaking or aperture correction. Peaking is typically applied to luminance signals and involves generating a processed high frequency version of the original signal and adding this high frequency version back to the original signal to get an enhanced signal. As shown in FIGS. 3 and 4 of Hitchcock, the problems associated with standard peaking techniques include excessive preshoot and overshoot (FIG. 3) and an emphasis on the high frequencies (FIG. 4) which can result in a noisy picture.
Rabii discloses a digital peaking method for processing chrominance signals and reducing noise. Rabii uses threshold decision circuitry and a digital comb filter structure to perform both transient improvement and noise reduction. Enhancement signals are calculated and output from a summation block 32 shown in FIG. 1. This enhancement output has the same basic form as the signal from a standard digital 2H comb filter that is commonly used for luminance enhancement, however, Rabii adds a limiter 54 to regulate the amount of preshoot and overshoot that is produced.
The enhancement equations required by peaking techniques are rather complicated, and accordingly, peaking techniques can be costly to implement. Also, the use of threshold decision circuitry makes it virtually impossible to respond to small amplitude chroma transients.
Additionally, preshoot and overshoot represent particularly difficult problems for chrominance signals because the lower frequency response of chroma causes any preshoots and overshoots to be even wider than with luminance. Accordingly, chrominance peaking is incorporated as a correction technique either very moderately or not at all in most television receivers.
U.S. Pat. No. 4,979,228 to Rzeszewski discloses a digital subsystem for improving resolution by inserting additional digital samples into a digital television signal. The Rzeszewski disclosure is directed primarily toward luminance signals, and the additional digital samples are calculated and added according to rather specific and relatively complicated algorithms and circuitry.
U.S. Pat. No. 4,729,014 to Flamm discloses a digital circuit for correcting poor chroma transients. Flamm uses a complicated slope detection technique that responds to digital chrominance signals that exceeds an amplitude threshold and are less than a time threshold . Flamm applies correction to the chrominance signals through a complicated switch arrangement that switches between an unmodified signal (FIGS. 1 and 2) and at least one other signal that has been modified by repeating previous sample values using a common digital video signal processing technique.
Aside from being complicated and costly to implement, Flamm also suffers from the inability to respond to small color transients (FIG. 4b). Additionally, the complicated decision circuitry and switching arrangement increases the risk that the switch will be set incorrectly (FIG. 3).
The present invention overcomes the problems associated with known methods of correcting abnormally slow chroma transients. In particular, the present invention provides the advantages of simplicity, cost effectiveness, and economy in a digital circuit and method for providing direct positive control over the rise and fall times of chroma transients. The present invention provides the further advantages of reliability, efficiency, versatility, and effectiveness for both high and low level color signals.